The present invention relates to a wiring board and a method for fabricating the same, more specifically, a wiring board including a conductive core material, and a method for fabricating the same.
Recently, electronic apparatuses rapidly have higher performance and are smaller-sized. The semiconductor devices forming the electronic apparatuses, and the wiring boards for the semiconductor devices to be mounted on are required to be accordingly small and thin and to have high performance and high reliability. To meet such requirements, as a semiconductor device mounting technology, bare chip assembly, which mounts semiconductor devices directly on a printed wiring board, is used.
As semiconductor devices have more pins, multilayer wiring boards including multi-wiring layers are more important. For tester boards for testing semiconductor devices, it is essential that the wiring layers are multilayered. As a multilayer wiring board, the built-up type, for example, in which fine wirings including insulating layers and conductor layers alternately superposed are formed on one surface or both surfaces of the core substrate is used.
In the bare chip assembly, silicon chips are mounted directly on a printed circuit board of a glass epoxy resin board. The thermal expansion coefficient of silicon chips is about 3.5 ppm/° C., while the thermal expansion coefficient of the glass epoxy resin board is 12-20 ppm/° C. The thermal expansion coefficient of the glass epoxy resin board for silicon chips to be mounted on is so largely different from that of the silicon chips. It is necessary to decrease the thermal expansion coefficient itself of the printed circuit board for silicon chips to be mounted on so as to prevent fatigue breakage, disconnections, etc. due to stresses caused by the thermal expansion coefficient difference.
As means for reducing the thermal expansion coefficient of the printed circuit board, carbon fibers having an about 1.0-20 ppm/K thermal expansion coefficient are used in place of the glass cloth used as the base material of the glass epoxy resin printed circuit board. However, epoxy reins are inexpensive and very universal as an insulation layer material but can usually have a thermal expansion coefficient of above 40 ppm/K including 40 ppm/K even by mixing an inorganic filler to thereby decrease the thermal expansion coefficient. Thus, the use of epoxy resin makes it difficult to sufficiently decrease the thermal expansion coefficient of the printed circuit board.
The background arts of the present invention are disclosed in e.g., Japanese published unexamined patent application No. 2002-084072, Japanese published unexamined patent application No. 2000-236167, Japanese published unexamined patent application No. 2002-100869, and Japanese published unexamined patent application No. 2000-133942.
In order to decrease the thermal expansion coefficient of the wiring boards used for the bare chip assembly, etc., recently, materials of low thermal expansion coefficients, such as carbon fiber, invar, etc., are used as the core materials of the wiring boards. However, many of these low thermal expansion coefficient materials are conductive, as is not the glass epoxy resin board. Accordingly, it is necessary to insulate the conductive core materials from the wirings of plated layers, etc., formed in the through-holes formed in the core material.
In the step of filling the through-holes by the conventional printing, the fillers, such as epoxy resin, etc., often become short. Voids, cracks, etc. are often made in the filler buried in the through-holes. Resultantly, the conductive core materials and the plated layers often short-circuit with each other.